The industrial manufacture of a variety of products involves the dispensing of liquids onto substrates or into containers. For example, completed printed circuit boards are manufactured by applying minute amounts of conductive epoxy at locations required for electrical connections between electronic components and the etching of the printed circuit board. Other well known liquid dispensing applications commonly include the filling of containers with measured amounts of liquid product.
In order to accomplish such liquid dispensing the conventional liquid dispensing apparatus heretofore provided generally includes a syringe barrel and a hypodermic needle. The syringe contains the liquid to be dispensed and the needle, attached to the syringe, is utilized for the point dispensing of the liquid. An air pressure source, supplying pressurized air, is connected to the syringe to force the liquid contained therein through the hypodermic needle and onto the substrate or into the container. In at least one liquid dispensing apparatus, suction is applied within the container by routing pressurized air through a venturi. The purpose of this is to prevent the drip of liquid from the needle between dispensing cycles. When dispensing is required, the pressurized air is re-routed directly to the container. The routing of pressurized air can be controlled by a solenoid air valve. Depending upon the accuracy of the metering required, the air valve is actuated either by a manually operated switch or by an automatic timer actuated by the manually operated switch. In many of the liquid dispensing apparatus, including that of concern here, the operator simply holds the syringe and directs the hypodermic needle to the desired location of application. Stationary fixtures as well as fixtures controlled by digital computers are also utilized in combination with such liquid dispensing apparatus to hold the syringe and to direct the hypodermic needle.
The problem with the conventional apparatus is that even where suction is provided to prevent drip, it is difficult to accurately meter the dispensing of minute quantities of liquids. There exist many reasons for this, but primarily since apparatus using suction require a finite time interval to switch from negative to positive pressure, the time interval for switching introduces an inherent limitation as to the smallest volume of liquid that can be accurately dispensed during a dispensing cycle.
One aspect of the present invention is therefore, to provide a liquid dispensing apparatus including an anti-drip valve cartridge, which can accurately dispense very minute quantities of liquids having a very low viscosity without drip from the needle between dispensing cycles. Moreover, as will be discussed in greater detail hereinbelow, since the present invention does not use continual suction to prevent drip, an apparatus constructed in accordance with the teachings thereof requires less energy than a dispensing apparatus of the prior art. Additionally, since the liquid dispensing apparatus of the present invention only operates on positive pressure, a plurality of syringe and needle combinations can be pressurized by a single air pressure sourse. The advantage of this is that a liquid dispensing apparatus as taught by the present invention can be advantageously incorporated into an assembly line manufacturing scheme with a low cost of installation and a further reduction of operating costs over the prior art.
A central feature of the subject invention that provides its advantageous operation is the anti-drip valve cartridge thereof. As will be described in greater detail hereinafter, the valve cartridge of the subject invention incorporates a casing having an outlet opening and means for sealably connecting the casing to one end of a needle element with the outlet opening in registry with the bore of the needle element. When the outlet opening is sealed, the needle element contains a volume of liquid remaining therein which exerts a pressure that is less than the incoming atmospheric pressure acting against the liquid at the other end of the needle element. In a preferred embodiment, a pressure activated poppet valve can be used for sealing and unsealing the outlet opening. When the container is pressurized, the valve opens and liquid flows from the end of the needle element. When the container is depressurized, the valve closes to seal the outlet opening. As a result, after the time interval for pressurization or the dispensing cycle, a minimum volume of liquid remains in the needle element which is operable to be acted upon by the incoming atmospheric pressure so that when the product of the specific gravity and the height of the column of liquid in the needle element is less than atmospheric pressure liquid will not drip from the needle element after the dispensing cycle. Moreover, in such a preferred embodiment, the connection means can include elements of standard luerlock fittings so that the valve cartridge of the present invention can be used with some of the conventional elements of liquid dispensing apparatus of the prior art.
While the prior art is repleat with pressure activated poppet valves, the prior art does not disclose a valve incorporating the features briefly described above. In this regard the prior art provides check valves, all having valve seats centrally located within their casings. Such check valves therefore do not teach, among other aspects of the subject invention, a casing having connection means that connect the casing to the needle element with the outlet opening thereof in registry with the bore of the needle element. An example of a check valve of the prior art is U.S. Pat. No. 2,538,364, entitled, "Valve," which issued to R. W. James ET AL on Jan. 16, 1951. James ET AL discloses a valve catridge having a centrally located valve seat and a valve within the casing thereof. Another example is U.S. Pat. No. 3,255,774, entitled, "Adjustable Inline Relief Valve," which issued to E. J. Gallagher ET AL on June 14, 1966. Gallagher ET AL provides a valve catridge having a pressure activated valve on the end of a hollow valve stem within a valve casing. The disclosed valve is biased against a valve seat in the inlet opening of the valve casing. U.S. Pat. No. 3,756,273, entitled, "Valve," which issued to Hengesbach on Sept. 4, 1973 again discloses a valve catridge having a valve and a valve seat, both of which are centrally located within a casing. A review of these references also discloses that none of them provide for the sealable connection between an outlet opening of a casing and a needle element. In this regard, U.S. Pat. No. 4,051,852, entitled, "Aspirating Device," which issued to Villari on Oct. 4, 1977 discloses a valve cartridge which can be positioned between a syringe and a needle element by provision of a set of standard medical male and female luerlock fittings. However, the disclosed valve is a spring loaded, ball check valve to prevent back flow from the outlet opening. The valve itself is, again, centrally located within the casing. It therefore, also does not disclose a connection means that could produce the anti-drip feature of the valve catridge of the subject invention. U.S. Pat. No. 2,845,066, entitled, "Syringe With Attached Serum Bottle," which issued to C. T. Hoppe on July 29, 1958 discloses a ball check valve sealing an opening in the end of a syringe. Obviously, Hoppe does not disclose a valve cartridge. Additionally, it does not teach the connection means of the subject invention and the anti-drip capability that is inherently provided thereby. Hence, besides the fact that such an assembly could not be used with conventional components of the liquid dispensing apparatus of the prior art, the needle element thereof would drip upon the relaxation of pressure within the syringe.